Magnetic nanoparticles, coercivity

Inorganic matrix can be used for magnetic nanoparticles assembly. Cobalt particles can be placed in the cellular structure of mesoporous silica [349]. The constraining in nanosize pores creates a special one-dimensional interaction between the magnetic material particles, resulting in increased coercivity. 

Fig. 7 Schematic representation of the evolution of the magnetic properties of magnetic nanoparticles as a function of their volume and of the models suitable to describe them. The label (1) illustrates that the maximum magnetic field for which the linear response theory (Neel relaxation model) is valid decreases with increasing volume. The label (2) is the domain where incoherent reversal modes occur so Stoner-Wohlfarth model based theories are not valid anymore. The label (3) shows a plateau in the volume dependence of the coercive field. Reprinted with permission from Ref 41. Copyright 2011, American Institute of Physics.

Hydrogen decomposition desorption recombination (HDDR) process is the only top-down industrial process used for the preparation of coercive nanoparticles. This process applied to rare-earth transition-metal (RE-TM) alloys consists in heating the concerned alloy under hydrogen until it decomposes into a fine mixture of RE-hydride and TM. The hard magnetic phase is recombined with a much finer microstructure. This process was first developed to convert 100 microns sized cast Nd2Fei4B grains into 200-300 nm crystallites [18, 19]. Later, it has been applied to other RE-TM alloys [20, 21]. Recently, a new variation of this process has been developed towards developing texture in the final materials [22], It is briefly described below. 

Even below TB, the magnetization of nanoparticles may be strongly affected by thermal activation. Very small Co particles showing coherent rotation allowed thermal activation effects to be analyzed quantitatively [110]. From 40 mK up to 12 K, the coercive field was found to be a function of the expression theoretically expected for thermal activation, Tln(T/T0]213. 

Magnetic properties of nanoparticles of transition metals such as Co, Ni show marked variations with size. It is well known that in the nanometric domain, the coercivity of the particles tends to zero. 23 Thus, the nanocrystals behave as superparamagnets with no associated coercivity or retentivity. The blocking temperature which marks the onset of this superparamagnetism also increases with the nanocrystal size. Further, the magnetic moment per atom is seen to increase as the size of a particle decreases 25 (see Figure 7). 

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